Conventionally, power factor (PF) is not considered in designing circuitry by almost all circuit designers. However, as to the design of power supply, since the input current thereof typically is a non-sinusoidal wave, in order to obtain a direct current (DC) voltage from an alternating current (AC) input end of the power supply, a bridge rectifier in parallel with a input filtering capacitor are typically provided in designing the power supply for converting and filtering an alternating current (AC) input. A charging on the input filtering capacitor occurs only when voltage of the input current exceeds a voltage across two terminals of the input filtering capacitor. In the conventional power supply, for reducing ripple factor, a capacitance of the input filtering capacitor is necessary to be designed as large as possible, which eventually causes the voltage of the AC input lower than that of the capacitor in most time. In other words, the bridge rectifier is conducted only in a short period of time of a half-cycle. As a result, a peak of the obtained waveform is several times larger than that of the equivalent root-means-square (rms). An undesired distortion of the power source is occurred due to an equivalent inductance as the very high instantaneous peak current flows through windings of the power supply. The distortion can increase a load of the power source. This is so-called power pollution. Theoretically, magnitude of the power pollution is expressed in terms of harmonic component or PF.
In fact, PF consists of current distortion and phase-shift parameters wherein the later can be compensated in the power source side. But the former has to be corrected in the side being used. For example, a load having a rated 15 A is electrically connected to a 15 A power socket of an AC power socket assembly having a nominal rating of 115V and 15 A. In a case that a power supply (having a PF of about 0.6) electrically coupled to the load does not have a power factor correction (P.F.C.) mechanism an effective input current as outputted from the power supply will only be 9 A not the desired 15 A. In view of the above, in a case that, in a normal operation, four computers all having P.F.C. circuits are electrically connected to the AC power socket assembly, there is no way to guarantee that two computers without built-in P.F.C. circuits can operate normally.
The power pollution caused by the distortion not only reduces an efficiency of power network thus causing difficulties of power control to a power company but also forces the power company to use thicker lines for power transmission. In this regard, recently many European countries set out a number of rules such as EN61000-3-2 for limiting a ripple current generated by power equipment and requiring all lights and electrical appliances having large power to be equipped with P.F.C. circuits prior to being permitted to import to Europe. Such rules must have a certain degree of impact on Taiwan since Taiwan is an export-oriented economy with household appliances and information products being the most important export items. Thus, it is important for manufacturers to install P.F.C. circuits in household appliances, computers, monitors, and power supplies prior to exporting the same to European countries since the P.F.C. circuit can effectively eliminate the undesired effect caused by ripple current.
In recent years, for complying with the current limitation of EN61000-3-2 many power supply manufacturers install power factor correctors (PFCs) in their switch-based power supply products. The PFCs may be classified as either passive ones or active ones. Booster type PFCs are the most widely used ones of active PFCs. Such booster type PFCs are further classified as either continuous conduction mode (CCM) ones having a fixed frequency or boundary mode ones having a variable frequency. A CCM controller is featured by a smaller peak current flowed through a switch, i.e., lower conduction loss. However, it has a larger switching loss and a poor EMI. Such CCM controller can be found in an IC having a Serial No. UC3854 available from Unitrode Inc. The IC has 16 pins and is complicated in applications. In contrast, a variable frequency-based controller is featured by a zero switching current, i.e., lower switching loss. However, it has a larger conduction loss and a higher peak current passed through the switch. Such variable frequency-based controller can be found in an IC having a Serial No. L6561 available from SGS Thomson Inc. or an IC having a Serial No. MC33262 available from Motorola Inc. Either IC has 8 pins and is simple in applications. It is important to note that the RDson parameter should be taken a particular interest in selecting a switch since the lower of value of the RDSon parameter the lower the switching loss.
In general, operation principle of a switch-based power supply is as follows: Stored energy is adjusted by adjusting a duty cycle of a switch, thereby outputting power. As such, the active PFC is featured that an input energy is adapted to output requirements during the duty cycle of switch adjustment and a shape of input current is adapted to be similar to the sinusoidal wave of AC power source. It is known that an electronic switch is activated only when current is zero under the PFC operating condition. Hence, a built-in zero current detector (ZCD) is required. This is best illustrated in a circuit diagram of a well known power supply 10 of FIG. 1. The power supply 10 has an incorporated PFC 20. A waveform of the power supply 10 is shown in FIG. 2. An operation principle of the power supply 10 is as follows: When a discharge current of an inductor 30 of a booster type switch is dropped to zero the stored inductance and stray capacitance will generate a harmonic. Voltage Vns of a secondary winding 31 of the inductor 30 will have a negative-edge waveform from high to low. Referring to FIG. 1 again, an example of the PFC 20 formed of the ICs having Serial No. L6561 available from SGS Thomson Inc. will now be described. When a ZCD pin ZCD detects that voltage VZCD has dropped below a critical voltage Vth a comparator in the PFC formed of the ICs having Serial No. L6561 will be triggered to generate a control signal for conducting the MOSFET switch 40. This is the mechanism of zero starting current. The detected voltage at pin ZCD must be larger than the critical voltage Vth before a next conduction in order to reset the PFC 20. Thus, a winding ratio n of the inductor winding relative to a winding coupled to ZCD pin ZCD (called ZCD winding in the invention) must satisfy the following equation (1):                     n        ≤                                            V              0                        -                                          V                                                      i                    ⁢                                                                                   ⁢                    n                                    ,                  rms                                            ⁡                              (                max                )                                              Vth                                    (        1        )            In a case that the power supply has a rated input voltage Vin in a range of 90V to 264V and a rated output voltage Vo of 400V the ratio of ZCD winding must be lower than 12.7:1. In a case that a ratio of the secondary winding 31 is 20:1 as designed the input voltage Vin will be 264V, resulting in abnormal discontinuous duty cycles of a control circuit. This is because when the input voltage Vin is equal to a peak Vipk, the voltage VZCD at the ZCD pin does not exceed the critical voltage Vth. As a result, the reset is disabled. The abnormal operation can be best illustrated by referring to the waveform of FIG. 3. As such, typically the secondary winding 31 must be reconfigured to comprise a ZCD winding 311 for current detection and a power supply winding 312 for supplying power as shown in FIG. 4(a). This not only increases a complexity of circuitry and manufacturing cost but also increases a size of the power supply. Moreover, one solution proposed by a manufacturer is that the single secondary winding 31 is shared by the ZCD pin ZCD and the power supply pin Vcc of the PFC 20. As such, the ratio of the secondary winding 31 is designed to comply with the requirements of the ZCD pin ZCD. Further, the secondary winding 31 is directly coupled to the ZCD pin ZCD. However, voltage of the secondary winding 31 after being rectified by the capacitor filter will exceed a required voltage at the pin Vcc of the PFC 20. Thus, an additional linear voltage stabilization circuit 32 is incorporated in the power supply 10 for lowering voltage as shown in FIG. 4(b). As a result, a reduced voltage of power supply is obtained. Unfortunately, such technique cannot reduce the complexity of circuitry and the manufacturing cost. Thus improvement exists.